1. Field of the Invention
This invention is directed to a chemical process for the treatment of industrial steelworks waste, especially those from an electric arc furnace, (EAF), called “flue dust”, to allow for the subsequent recovery of zinc, iron and other interesting metals contained in the flue dust.
2. Description of Related Art
The zinc has always been used as a protective coating against corrosion on ferrous metals, thus generating the so called “galvanized metal”, which, in many cases, substitutes stainless steel items like plates, roofing, screws, and tubes.
Most of the ferrous metals or metal alloy including common iron, carbon steel, alloy steel and cast iron, are recycled several times, creating a recovery cycle of the metal through the reutilization of the scraps as a source of raw material. Thus, all zinc applied in the galvanization of ferrous metals undergoes the same process. However, when these scraps are recycled, the zinc in the scraps is separated from the iron, as a result of the high temperatures in the steelworks furnaces. This separation happens because the melting point of the zinc is lower than that of the iron, so the zinc is volatilized and pulled out of the furnace through the special electrostatic or sleeve filters along with the other dusts from the furnace. This residue is called “flue dust”. Since this dust is generated in steelworks that recycle scrap by using electric furnaces, also known as electric arc furnaces, the dust may also be called “electric arc furnace dust ”(“EAF”).
This residue contains average percent values of about 20% of zinc, considered here in the elementary form (Zn) and about 28% of iron, also considered in the elementary form (Fe), combined in a chemical structure know as “zinc ferrite”, the formula of the combined oxides is ZnO.Fe2O3. Besides these, lead (1.5%), chrome (0.25%), cadmium (0.05%), tin (0.15%) and other elements in less contents, such as sulfur, manganese, copper, calcium, magnesium and nickel (generally as oxides) are also found in its composition.
Furthermore, significant amounts of fluorine (F) and chlorine (Cl) are also found in the flue dust. The Chlorine (Cl) comes from the plastics contained in the scraps which are very likely to be combined in carbonic structures called dioxins. Such substances when added to the heavy metals (such as e.g. lead, chromium, and cadmium) classify the flue dust as “dangerous”, making its disposal in controlled filling compulsory. This restriction causes the steelworks cost to go up significantly, generating in turn significant amounts of rejects per ton of produced steel, without there being definitive and environmentally correct solutions. Many attempts have been presented for the reuse of the flue dust, however few are economically feasible. Several technical barriers impede the processing of the flue dust. Currently, very few recycling processes have been consolidated in efficiently in the economic and technical perspective. As of late, one of the most well-known and used process is the Waelz Process. This process, nevertheless, requires heavy investments, large production scale, and generates new residues thus, being considered environmentally unsustainable in some countries.
The current status of hydrometallurgical technique for the extraction of metals from ores and industrial residues, acid or alkaline liquid means for dissolution of the oxides, hydroxides, carbonates, silicates and sulfides that contain the interest metal are normally used. The traditional applications of the hydrometallurgy include the production of alumina, gold, uranium, zinc, nickel, copper, molybdenum, titanium, and rare earth, among others.
In the first phase of the hydrometallurgy, the physical-chemical properties of the solid, such as the particles size, composition, content, chemical nature and porosity are adjusted for the next phase, called “the leaching”. This first phase consists in an extraction process of a substance from a solid medium by means of its dissolution in an acid or alkaline liquid that reacts chemically with the elements present in the ore or residue, originating a new soluble substance. The hydrometallurgy is greatly used in several fields of science, such as geology, metallurgy, and chemistry. Its preparation involves classical ore treatment operations, like the comminuting or grinding, classification, concentration and the solid-liquid separation.
Once the ore is prepared, the leaching phase begins. These first two phases are the most characteristic phases of the hydrometallurgical flowchart. The leaching makes the selective dissolution of mineral containing the metal or metals of interest, promoting the contact of the solid formed by the concentrate ore or the industrial residue with an aqueous phase containing acids. The acid found here are usually the sulfuric or muriatic acids, bases, like ammonium or sodium hydroxides or complex agents, like sodium cyanide and ammonium hydroxide, with temperatures varying from 25° C. to 95° C. and under varied pressure. This phase is followed by the solid-liquid separation operations, using processes such as cyclone, thickening and filtering, with the purpose of reaching the aqueous or liquor phase that contains the metals of interest. The efficiency of this phase is determined by minimizing the losses of soluble metal in the pulp, which constitutes the waste, and by the consumption of fresh water in the process.
The characteristics of the solids that are being discarded are also determined by the costs of the disposing of the reject and the potential risk of environmental impacts. The treatment phase of the liquor produced in the leaching is designed to purify the solution through the separation of elements coming from the dissolution of the ore or residue that may affect the subsequent phase of the metal's recovery. Secondly, it is designed so that the concentration of the solution containing the dissolved metal reaches adequate levels for the recovery phase which follows. Eventually, this phase may lead to obtaining secondary products.
The treatment of the liquor involves processes such as e.g. precipitation, adsorption in activated coal or in polymeric resins of ionic change and extraction by solvents. The processes employed in this phase may be applied to the treatment of effluents, aiming to concentrate and remove the contaminating elements.
The last phase of the hydrometric flowchart is designed to recover the metal. The metal may be obtained in salt or metallic hydroxide form, as Al2O3.nH2O and CuSO4 (through the precipitation and crystallization processes) or as metallic form. When in metallic form, reaction reduction in aqueous phase is used, such as e.g. cementation, which is the reduction by oxidation of a less noble metal. The reduction by means of hydrogen or the electro recovery is also used, which is the main process used in the production of metals of high purity directly from aqueous solutions. The process comprises the application of a potential difference between cathodes and anodes immersed in aqueous solution and it is used to obtain copper, zinc, nickel and gold, among others. For metals of very negative redox potential, like aluminum, the electro recovery is made in molten salt bath.
However, some difficulties are found for the viability of this process. Several ores or residues present significant resistance to leaching, that is, they are not attacked by the acids or bases even when exposed to high temperatures and high concentrations of leaching agents. For these cases the status of the current technique suggests pretreatment like: (i) the reduction, which consists of the heating of the ores or residues in an environment reducer with temperatures above 1,000° C. with coal, coke or products containing carbon, which are useful for the seizure of the oxygen present in the ores, found as Oxides; (ii) the ustulation, which, in summary, is the heating of ores containing sulfides at temperatures above 600° C. for oxidation of sulfide ores and the liberation of sulfur dioxide; (iii) the pressure leaching, which is the oxidation under pressure and high temperature that may reach up to 250° C.; and (iv) the biohydrometalurgy, which is the biological oxidation of refractory ores by using microorganisms.
This additional treatment makes the costs of equipment and reagents increase significantly, causing many times the technical unfeasibility. Because the flue dust is very refractory to the acid attack and the alkaline attack, a process previous to the leaching is necessary. The most well-known and used around the world nowadays is the two-phased processes called “Waelz”. The first phase or pyrometallurgic phase, happens in direct flame rotating kiln. The flue dust previously mixed coke breeze or mineral coal is briquetted and reduced at temperatures around 1,200° C. The next step is the distillation of the zinc which is carried to the electrostatic filters by the exhaust gases and later oxidized, a process also known as “Oxiwaelz”. The second phase comprises the removal of the filtered material which goes on to the hydrometallurgical separation and concentration phases until the metallic zinc purified in electrolytic process can be obtained. The other metals form a mass or slag made up of iron and the heavy metals which then go on to fillings or immobilization in Portland Cement Industries. The zinc recovery index in this process is above 93%, however, the process is applied only in large scale due to the heavy implantation and production costs and to the complex number of hydrometallurgical phases, possible dioxins removed by the temperature of the pyrometallurgic phase, which can be regenerated in the cooling, thus maintaining the original problem.
The processes that only use hydrometallurgy for processing the flue dust usually do it with alkaline leaching using sodium hydroxide or ammonia. Nevertheless, the recovery rate of these methods is too low, being in some cases under 50%. Furthermore, the liquid effluents generated in the process must be considered, which have to, in turn, undergo treatments before being discarded.
U.S. Pat. No. 5,538,532 describes a method for the separation and recovery of metals selected from the group consisting of iron, cadmium, zinc, and lead, from raw material comprising a mixture of metals, which comprises the steps of heating the raw material to a temperature sufficient to substantially vaporize cadmium, zinc, and lead, and insufficient to substantially vaporize iron; separating secondary dust and vapors produced during the first step from the residual sinter mass, which mass comprises iron; slurring the secondary dust in an aqueous solution of ammonia ammonium carbonate to dissolve zinc and cadmium; separating a zinc/cadmium bearing leach liquor from substantially insoluble lead containing particles by filtration; treating the zinc/cadmium hearing leach liquor to recover cadmium by adding metallic zinc to the leachate to produce a cadmium containing cement; separating the cement from the leach liquor; and removing ammonia from the leach liquor to precipitate basic zinc carbonate. This method presents a process quite different to the present invention, as it employs high temperatures and, mostly, pyrometallurgy principles.
U.S. Pat. No. 4,614,543 discloses a process for the hydrometallurgical treatment of finely divided iron-containing steel plant dusts containing zinc, lead and such other metal values as calcium, manganese, silicon, magnesium, aluminum, cadmium, copper, and the like. The process is carried out by forming an aqueous slurry of the flue dust with a mixed lixiviant comprising HCl and H2SO4, the amount of sulfate ion concentration being in excess of the chloride ion concentration and in stoichiometric excess of that required to sulfate substantially all of the lead and calcium present. The amount of chloride ion present as HCl should be sufficient to maintain the pH at about 1 to 4. The leaching is conducted at a temperature ranging from ambient to below the boiling point for a time at least sufficient to effect dissolution of at least zinc and other metal values and form a residue containing iron oxide, calcium sulfate and lead sulfate. The method regards a process characterized by the use of sulfuric acid and some hydrometallurgical operations which are quite different of those presented in this invention.
U.S. Pat. No. 4,915,730 discloses a process and apparatus for the recovery of metals such as silver from phosphate flue dust. The process includes the steps of blending chloride salt and the flue dust to produce a blended material, roasting the blended material in an oxygen bearing atmosphere to oxidize carbon in the blended material producing a gas and to react chloride salt with the metal in the blended material producing a water soluble metallic salt, dissolving the metallic salt in water to produce a solution, filtering the solution to remove solids, and precipitating metals from the filtered solution with the precipitate ready for conventional smelting. The preferred embodiment of the apparatus includes a flue dust hopper and mill and a salt hopper and mill for feeding the dust and salt to a radiant tube dryer and a radiant tube asher for blending and roasting the materials, and a spray chamber at the outlet of the asher for separating solids and gases, where certain of the solids go into solution. The apparatus further includes a filter for removing the undissolved solids, a zinc feeder to add zinc to precipitate the dissolved silver, and a filter for removing the zinc-silver precipitate which is ready for smelting. The methods regards a slightly similar process, as it happens at substantially higher temperatures, employing partial fusion and reactants different from those proposed in this invention.
WO/1994/019501 discloses a process for the treatment of electric arc furnace (EAF) dust, the dust is first subjected to atmospheric leaching with a ferric chloride solution and thereafter subjected to treatment in an autoclave at an elevated temperature and pressure for conversion of low temperature stable goethite (FeO.OH) to a filterable crystalline hematite (Fe2O3) in an acidic chloride solution. Zinc is recovered from the solution by solvent extraction using a solvating extractant followed by stripping and zinc recovery by electrolysis of zinc chloride or zinc sulfate solution. Lead is separated from the solution by cooling to precipitate lead chloride. The method regards a process which is quite different from the one proposed here, employing hydrometallurgical means making its operations in liquid medium and with different reagents than those of the present invention.
U.S. Pat. No. 4,355,009 discloses a hydrometallurgical process for separate treatment of zinc-bearing metallurgical flue dust containing significant amounts of lead, chlorine, and iron. The process is especially suited for extraction of zinc sulfate from blast furnace white dust resulting in the smelting of secondary copper. According to the process, the flue dust is leached in sulfuric acid solution for substantially complete dissolution of soluble constituents, notably zinc, leaving insoluble residue consisting principally of lead oxide. At completion of leaching, pH is selectively adjusted corresponding to the desired extent of subsequent chloride removal. Second, the loaded leach solution is treated for chloride removal wherein chloride ion concentration is substantially and selectively reduced by precipitation of cuprous chloride, cuprous ions being provided by pH regulated reduction of cupric ions. Third, the de-chlorinated leach solution is treated by pH regulated cementation with zinc to remove residual cupric copper from the previous step along with other metal impurities more noble than zinc. Fourth, iron is precipitated from the acidic leach solution by oxidation of acid-soluble ferrous ions to the relatively insoluble ferric state. Finally, the purified leach solution is subjected to evaporative crystallization to recover commercial grade zinc sulfate. This method is yet again quite different from the one proposed here, being distinguished only by the fact that it also proposes the formation of soluble salts or sulfates, substantially different from the proposition of the present invention.
U.S. Pat. No. 5,961,691 describes a process for extracting and recovering lead or lead derivatives in high purity from various materials containing lead sulfate, and particularly copper smelter flue dusts. The present process also allows the substantially complete recovery or recycling of precious metals otherwise lost in flue dusts. The method regards a process non-similar to the present invention.
U.S. Pat. No. 3,983,218 describes a process using run-of-the-mill flue dust from open hearth and basic oxygen steel making processes having certain iron-zinc values as a sulfur dioxide absorbent and pollutant control for industrial and public utility furnace flue gases, which materials, upon solid dry injection into a chemical reaction zone of an industrial or public utility furnace in an amount in excess over stoichiometric, results in substantial dry removal of the sulfur dioxide there from, and from which reaction products are cleaned by conventional gas cleaning apparatus. The method regards, then, a process different to the present invention.
U.S. Pat. No. 4,119,455 discloses a method of recovering iron-bearing flue dust collected as a by-product in wet sludge or dry form from metallurgical processes for recycling. The moisture content of the collected flue dust is adjusted to a level at which the wet dust is of a plastic consistency such that it will extrude into cohesive agglomerates (generally 8-16% moisture content). If the dust is collected in a dry state, moisture is added; if collected in a wet state, the moisture content is adjusted by the addition of a complimentary dry material. Hydraulic cement is added to the mixture in the range of approximately 4-15% by weight and the mixture is extruded into cohesive agglomerates and thereafter cured for subsequent charging to metallurgical furnaces. The method proposed remains unique when compared to yet this invention.
What is needed is a recycling process that is economical, technically less complex and optimized for the environmental conditions.